This invention relates to a method of laminating molded articles formed of thermosetting melamine-formaldehyde resin and comprising an indicia-bearing opaque sheet, such as a photograph or other decorative surface, and to the laminated articles thusproduced.
Laminating layups for structures, such as plaques or plates have been made by the following techniques:
(A) A map, print or other decorative surface is obtained on the thinnest commercially available printing paper and coated with an aqueous solution of a laminating resin. After removal of solvent, the impregnated surface is heated to partially cure the resin to a condition suitable for thermosetting laminating techniques, in which the resinized decorative surface is placed over a resin underlay of resin-impregnated cellulosic paper and the layers are fused together by heat and pressure.
(B) Instead of precoating the decorative surface with laminating resin, the resinized protective overlay and the base underlay are processed to contain excess resin sufficient, under cure conditions, to flow into and partially penetrate and laminate the decorative surface.
(C) An absorbent sheet is impregnated with resin and cured to a condition suitable for laminating, whereupon the design is printed thereon by silk screen or flat press printing to avoid cracking the brittle resin-impregnated sheet. A composite of protective overlay, printed paper and base paper is then cured.
Deficiencies of these methods include a high reject rate and poor quality laminates, as indicated by blistering and surface crazing during heat testing, by partial delamination of the products, and by warpage of the composites.
An improved layup, disclosed in U.S. Pat. No. 3,537,950, uses an overlay sheet of 62-77% resin pick-up of melamine-formaldehyde resins and a base cover sheet of 35-55% resin pick-up to surround a porous inlay, e.g., .alpha.-cellulose paper. However, the application of this method is limited to cases in which the inlay has the requisite porosity characteristics.
Another improved process, disclosed in U.S. Pat. No. 2,833,685, requires treatment of papers, prior to offset lithography, with melamine-formaldehyde resin on one side thereof. Although sheets thus-treated are pliable and can be handled in conventional lithographic presses, the printed material must be further impregnated with resin prior to the final lamination step.
It will be apparent that all of the foregoing methods have deficiencies or require specialized materials. It would be highly desirable to provide a process which permits the fabricator to employ solutions of melamine-formaldehyde resin and melamine-formaldehyde powdered molding resins which are inexpensive, commercially available and can be reliably employed to fabricate custom-made items from colored or black and white photographs, etc.